Electric heating



United States Patent O ELECTRIC HEATING Lester D. Drugrnand, Pittsburgh, Pa., assignor to The Edwin L. Wiegand Company, Pittsburgh, Pa., a corporation of Pennsylvania My invention relates to electric heating elements, and the principal object of my invention is to provide new and improved elements of thistype.

ln the manufacture of tubular electric heating elements, the terminal ends of such elements are required to be effectively closed against the entrance of deleterious substances which would affect proper operation of the element. A requirement of such closure is to improve the electrical properties of the terminal ends of the heating element by increasing the electrical surface creepage distance from the terminal pin to the sheath. Also, it is desirable to replace air between the pin and sheath with a solid dielectric which will withstand higher dielectric breakdown potentials.

A common terminal end construction or closure is in the form of a dielectric bushing which is disposed over the terminal end of the resistor conductor and positioned within a pocket formed by stopping the refractory material short of completely filling the terminal end of the tubular sheath. A cement is commonly used to lock the bushing in place and to ll the space between adjoining surfaces; and, heretofore, this cement, because it is preferably of the air-dry type, was made in batches by mixing dry powder with a moistening agent (usually water), and the resultant plastic mixture was placed, by various means, into the pocket formed at the terminal end of the tubular element.

One type of air-dry cement heretofore found suitable was formed by mixing 63.8% 60 mesh electrical` grade zircon, 21.87%-400 mesh electrical grade Zircon, 1.75% micronized (very finely divided) electrical grade Zircon, 4.72% mono magnesium phosphate (MgH4(PO4)2), and 7.86% magnesium oxide (MgO). This powder was mixed with about water by weight whereupon it would set hard in a relatively short time as the water evaporated.

The foregoing method was not satisfactory because of the diiculty in placing the plastic cement in to the sheath pocket, particularly in cases of elements having sheaths of relatively small diameters. cement was placed in the sheath pocket, great care had to be exercised and this materially affected production rate. Further, the cement had to be made in small batches since in large batches a lot of it was wasted because it would set before all of it could be used.

My invention overcomes vthe diiculty of placing the plastic cement within the sheath pocket and, at the same time, materially reduces production time heretofore required to produce an effective terminal closure.

in the drawing accompanying this specification and forming a part of this application, there is shown, for purposes of illustration, an embodiment which my invention may assume, and in this drawing:

Figure 1 is a separated fragmentary sectional view showing a dielectric plug ready to be inserted into the terminal portion of a tubular element, and

Figure 2 is a fragmentary sectional view showing the parts of Figure 1 in assembled relation.

To insure that suiiicient' "Ice The usual tubular electric heater comprises a tubular sheath 10, preferably of a suitable metal, a resistance conductor 11, here shown tobe in the form of a helical coil of resistance wire, and material 12 serving both to electrically insulate the conductor from the sheath and to conduct heat from the conductor to the sheath.

The material 12 is usually granular refractory rnaterial and may be introduced into the upper open end of an upright sheath in any suitable manner. In some forms of commercial production, a plug is placed in the lower end of the sheath and the refractory material is caused to flow into the open upper end, the sheath preferably being subjected to vibration to assist the refractory material in its flow and to provide some compactness to the refractory material within the sheath.

The filling operation is stopped before the sheath is completely filled or, if the sheath is completely filled, a certain amount of refractory material is routed out of the upper terminal end of the sheath to a level, for example as shown by the reference 14, to provide a pocket 15. It will be appreciated that removal of the plug in the lower end of the sheath will provide a pocket generally corresponding to the pocket 15. `insofar as this invention is concerned, the material 12 may take a form other than granular refractory material such as, for example, dielectric bushings disposed about the resistance conductor and with end bushings stopping short of each end of the sheath to form pockets.

In usual construction, each end of the coiled resistance conductor 11 is mechanically and electrically secured to a terminal pin 16. Each terminal pin preferably extends through the material 12 and through the pocket 15 to a point beyond the end of the sheath to provide for later electrical connection.

Heretofore, in closing the ends of the tubular element, a plastic mixture of cement powder and water was placed in the pocket 15, and a dielectric bushing 17 was placed in the pocket around the adjacent end of the terminal pin 16, the plastic mixture being sufliciently displaced by the bushing to at least partially enter the space between the terminal pin 16 and the opening 18 in the bushing 17, as shown by the heavy line 19 in Figure 2, and to enter the space between the peripheral surface 20 of the shouldered bushing and the inner peripheral surface of the sheath 10, as shown at 21 in Figf' ure 2. The cement was usually of the air-set type and the finished element was thereafter set aside for the cement to dry.

This previous method above described was slow because it was difficult to place the plastic cement into the pocket, especially in small diameter heating elements. Further, in the deposition ofthe plastic cement, a certain amount of the cement would tend to be spread in unsightly manner about the exterior of the sheath; and` in many cases, this cement had to be removed.

In use of my invention, I place a relatively dry cement powder (indicated by the reference numeral 22), which may comprise the composition heretofore disclosed, into the pocket 15, this dry powder being easily inserted in any suitable manner, such as by vibrating it into the pocket in a manner. similar to the manner of depositing the refractory material 12.

4After a suitable amount of powder has been placed into the pocket, a bushing 17 is placed over the terminal pin 16 and into the pocket an amount which may be limited by abutment of the bushing shoulder 23 with the terminal marginal surface 24 of the sheath 10. The bushing 17 is preferably formed as a ceramic body, or of any other suitable dielectric material which is some what porous and will carry moisture.

Moisture is added to the dry cement powder in any suitable manner and, at the present time, it has been found preferable to moisten the bushing by placing it in a Water bath prior to its deposition into the pocket i5. When the moistened bushing is properly disposed within the pocket, it has been found that the moisture carried by the bushing will' wet the dry cement powder sufiiciently to make a plastic composition, at least in part, which will flow enough to enter the spaces 20 and 2l and adhere to the surfaces defining such spaces. The tinished element may then be set aside to permit the cement to harden. lt will be appreciated that other methods may be used to Wet the dry cement powder, such as lby wetting the bushings l? after they are disposed within respective pockets and relying on capillary action to travel the moisture to the powder.

After the cement has hardened, the heating element may be subjected to various other steps of manufacture. A usual step is toA subject the element to a rather high temperature to anneal the metal sheath and/or bake the element to eliminate moisture from therewithin. It has been found that the annealing and/or baking steps sinter the dried cement to a somewhat ceramic form, thus imparting greater mechanical strength to the closure or terminal end construction.

Through use of my invention, production of tubular heating elements has been greatly facilitated without sacrifice in the quality of the closure or terminal end construction. The dry powder, being easily introduced into the pocket, may be regulated with a greater degree of exactness and will not adhere in an unsightly mass to the exterior of the sheath. Further, the annoying practice of mixing small batches of cement is eliminated.

In view of the foregoing it will be apparent to those skilled in the art that I have accomplished at least the principal object-of my invention and it will also be apparent to those skilled in the art that the embodiment herein described may be variously changed and modified, without departing from the spirit of the invention, and that the invention is capable of uses and has advantages not herein specifically described, hence it will be appreciated that the herein disclosed embodiment is illustrative only, and that my invention is not limited thereto.

I claim:

The method of producing an electric heater having a resistance conductor disposed in a tubular sheath and surrounded by an electric insulating heat-conducting material, comprising disposing saidconductor and said material within said tubular sheath inposition wherein a terminal end portion of said conductor is disposed in spaced lateral relation with an end opening of said tubular sheath and said material stops short of completely filling said sheath to provide an open pocket therein adjacent said end opening, placing an essentially dry cement powder in said pocket,` and disposing a moistened bushing having dielectric and hygroscopic properties over said terminal end portion of said conductor and into said pocket in generally closely itting relation with the inner peripheral wall of said sheath end opening and the adjacent exterior surface of said terminal end portion, the moisture contained by said bushing wetting said cement powder sufiiciently to cause it to adhere to adjoining surfaces of said sheath end opening and the terminal end portion of said conductor.

References Cited in the file of this patent UNITED STATES PATENTS 1,779,822 Noguchi oct. 2s, 1930 1,992,787 Sl1tto11. ..V Feb. 26, 1935 2,480,903 Charbonneau Sept. 6, 1949 2,527,890 Pouchnik Oct. 31, 1950 

